1. Field of the Invention
The present invention relates to a radio frequency power amplifier used in cellular phone handset, and more particularly to a voltage power supply method of reducing mutual interference between circuits through a power supply line.
2. Description of the Prior Art
Recently, there has been an increasing demand for cellular phone handset as typified by cellular phones to be more compact in size and lighter in weight, and therefore research and development is being vigorously made to satisfy the demand. Conventional power amplifying circuits for transmission used in cellular phone handset require negative voltage power supply or negative voltage power creation circuit, have many part configurations, and cannot therefore meet the demand of being compact in size and light in weight. Accordingly, heterojunction bipolar transistors of Gallium Arsenide compound semiconductor (hereinafter simply referred to as GaAsHBT) are expected as amplifying devices used in radio frequency power amplifiers for transmission because they are capable of single positive voltage power supply and have excellent radio frequency characteristics.
FIG. 12 shows a conventional radio frequency power amplifier employing GaAsHBT, disclosed in Japanese Patent Laid-Open No. H10-75130. The radio frequency power amplifier comprises: an input impedance matching circuit 401; a power amplifying transistor 410; an output impedance matching circuit 402; and a base bias voltage control circuit 403 for the power amplifying transistor 410. The base bias voltage control circuit 403 comprises a transistor 411 and resistors 420 and 421. The reference numeral 430 denotes power supply for driving the collectors of the transistors 410 and 411, and the reference numeral 431 denotes power supply for controlling the gain of the radio frequency power amplifier, which is applied to the base of the power amplifying transistor through the base bias voltage control circuit. The base bias voltage control circuit, by substantially supplying a base current Ibb of the power amplifying transistor from the power supply for collector driving, is generally used to reduce a current Iapc for gain control to be supplied from the power supply for gain control and thereby reduce current supply capacity demanded to external control circuits to generate a voltage for gain control.
With saturation amplifiers complying with the GSM (Global System for Mobile Communication) system widely used principally in Europe presently, a trade-off relationship between output power and power added efficiency is a major problem in the development of radio frequency power amplifiers.
Moreover, with linear amplifiers complying with the W-CDMA (Wideband Code Division Multiple Access) system, which is one of third generation mobile communication systems, in addition to the trade-off between output power and power added efficiency, a trade-off relationship exists between distortion and power added efficiency, as described in page 36 in xe2x80x9cElectronic Technology June, 2000xe2x80x9d published by Nikkan Kogyo Shinbun.
Therefore, in the linear amplifiers, reduction in distortion leads to an increase in power added efficiency of radio frequency power amplifiers, which, in turn, leads to an increase in the performance of the radio frequency power amplifiers.
In the above conventional radio frequency power amplifier, the power supply for driving the collector of the transistor making up the base bias voltage control circuit is shared with the power supply for driving the collector of the power amplifying transistor. The above conventional radio frequency power amplifier has no capacitor enough to cut off a high frequency leakage signal of the collector current Icc due to output power of the power amplifying transistor, e.g., capacitor enough to couple a collector line and the ground. Consequently, the high frequency leakage signal of the collector current Icc is fed back to the base current Ibb of the power amplifying transistor through the base bias voltage control circuit, causing the radio frequency power amplifier to operate unstably.
Moreover, the high frequency leakage signal of the collector current Icc is fed back to the base current Ibb of the power amplifying transistor through the base bias voltage control circuit, causing an increase in distortion in the output power of the radio frequency power amplifier.
An object of the present invention is to provide a radio frequency power amplifier of multi stage amplifying method that is designed to reduce instability of output power caused by electromagnetic coupling of bias supply terminals and inter lines of each stage to thereby operate stably. Another object of the present invention is to provide a radio frequency power amplifier of multi stage amplifying method that is designed to reduce distortion of output power caused by electromagnetic coupling of bias supply terminals and inter lines of each stage to thereby provide high efficiency characteristics.
The above objects can be achieved by providing the radio frequency power amplifier of the claims 1 to 7 with the first terminal for supplying a voltage for collector driving to the first transistor for power amplification, the second terminal for supplying a voltage for collector driving to a second transistor controlling a base bias voltage of the first transistor, the first interconnection for connecting the first terminal and the collector of the first transistor, the second interconnection for connecting the second terminal and the collector of the second transistor, and one or more ground parts for electromagnetic shield, wherein the first terminal and the second terminal are separated by one or more of the ground parts for electromagnetic shield, and the first interconnection and the second interconnection are separated by one or more of the ground parts for electromagnetic shield.